Joseph Bataille wrote:.... transition a fully mature forest into a fully mature food forest
What do you mean when you say "fully mature"? When I think of a "fully mature" forest, I think of old growth, and I'm not aware of anywhere that you can just buy some old-growth forest and move in and do what you like with it. The forests around where I'm at are usually only 50-100 years since their last bad fire or clearcut when people start casually referring to them as "mature", but even then I don't think they're nearly as developed as the "mature" forests of most of the planet's history.
All quibbles about the definition of "maturity" aside, I've faced similar difficulties to what you describe in that there don't seem to be a lot of resources for how to turn any non-food forest into a food forest in general. I'm kind of playing it by ear in my own projects, but one thing I think about a lot is my role of accelerating processes that would happen on their own in the forest if it was given enough time. For instance, conifers would eventually fall over, either individually or in groups due to freak weather events. Animals would eventually haul in seeds from edible plants, and so on. But part of the bargain between us humans and the plants we've selected to be great eating is that we take actions to give those plants a bit of an advantage, and in return they give us stuff to eat. If you accept humans as being parts of nature ourselves, there's nothing unnatural about us helping out the plants we like more than others.
The land I'm currently working with used to be a homestead about 100 years ago, and has been managed as timber most of the time since then. I've found feral plants that clearly descended from plants that the homesteaders brought in -- there's a thicket of some sort of plums that go from inedibly sour to all gone in about a day, and a couple apple trees that probably grew from seed or rootstock because the fruit tastes like cardboard and the trees have 2" spines, and hawthorns all over the place despite there being none in other nearby sections of forest. Nobody put these individual plants where they are or cared for them; their presence is just a side effect of what happens in a forest when people introduce food plants and then quit taking care of them.
If the flow of permaculture starting from a field is "build soil then grow a forest", the flow of permaculture starting from a forest seems to be "find light and then grow food in it". Field-started permaculture has unlimited light and tends to be badly constrained on soil; forest-started permaculture in my limited experience so far seems to be the opposite: A happy forest has soil that's hospitable to most trees without amendment, but intervention is needed to start little food trees out in locations where they have a chance to succeed. Additionally, wild-ish and timber-managed forests select for the fastest-growing trees instead of the tastiest ones, so food trees tend to need a bit of help keeping enough light throughout their lives.
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